This grant application is devoted to determining whether the blood coagulation system is involved in the pathogenesis of the underlying organ damage caused by sickle cell disease and to evaluating whether therapeutic approaches directed at the coagulation system suppress these complications. In our view, these issues can be most effectively resolved at the present time in murine models of sickle cell disease where detailed investigations of thrombosis and sophisticated manipulations of genes can be carried out. We will initially characterize mice exhibiting moderate intensity sickle cell disease with regard to the extent of coagulation system activity, the severity of thrombotic events, and the magnitude of organ dysfunction. The above studies will be carried out in a wild type background as well as in backgrounds with fibrinolytic pathway defects of increasing severity. The correlation of time dependent changes in the extent of coagulation system activity, the severity of thrombotic events and the magnitude of organ dysfunction may establish a linkage between the hemostatic mechanism and organ damage. We will then investigate the proposed linkage in mice with mild intensity or severe intensity sickle cell disease. We may observe strong correlations between the above three variables at all intensities of sickle cell disease or at a particular intensity of sickle cell disease or note the absence of correlations at any intensity of sickle cell disease. These data should allow us to ascertain whether the coagulation system plays a major role or is of little importance in the development of thrombotic events and organ dysfunction. We will then alter coagulation system activity by genetically modifying thrombomodulin function. We expect that these manipulations will lead to an augmented or suppressed development of thrombotic events and organ damage. This effect might occur in the presence of a strong correlation between the three variables outlined above at all intensities of sickle cell disease or in the absence of a strong correlation between the three variables at any intensity of sickle cell disease. Thus, these results could reinforce our view that the coagulation mechanism plays a major role in the development of thrombotic events and organ damage or suggest that a genetic abnormality of the coagulation system could act in concert with sickle cell disease to produce a dramatic acceleration in organ dysfunction. We will then ascertain whether administration of anticoagulants to mice with sickle cell disease suppresses thrombotic events and organ damage. The amounts of anticoagulants employed will be determined with sophisticated assays of the coagulation system in order to minimize drug dosage and maximize therapeutic effect. The murine models utilized will be critically dependent upon previously establishing conditions under which the coagulation system plays a major role in the development of organ damage. The test systems may include mice with various intensities of sickle cell disease or a particular intensity of sickle disease in a wild type backgrounds or in backgrounds with specific defects of the coagulation system.